Source: USA Today (thanks to 80schild.com for the list) and AP

1. NBC 16.0/26
2. ABC 13.7/22
3. CBS 13.5/22

New shows: My Two Dads, Day by Day, Frank's Place, Eisenhower and Lutz, Wiseguy, The Wonder Years, Full House, A Different World...

http://www.apnewsarchive.com/1988/List-of-Season-s-Top-Rated-TV-Shows-With-AM-TV-Ratings-Bjt/id-53b06e0c8df47c0ee910cfcc5c903b3f

What's the explanation for My Two Dads appearing twice with different figures?

What's the explanation for My Two Dads appearing twice with different figures?

I don't know, but there are more examples when different figures are reported for same shows

Frasier S1 - 17.5 HH (TV Guide), 17.0 HH (wiki)
Home Improvement S3 - 21.8 HH (TV Guide), 20.4 HH (wiki)
A Different World S1 - 24.9/39 (TV Guide), 25.0/40 (AP)
Mississippi (S1) - 19.4/35 15th place (Broadcasting)

The 1987-88 season marked the first time CBS finished in third and last place among the Big 3 networks.

What's the explanation for My Two Dads appearing twice with different figures?

Maybe it was airing two different nights?